About

Daniel Apai is a Professor of Optical Sciences and serves as the Associate Dean for Research at the College of Science at The University of Arizona. His research interests include space telescopes, astronomical instrumentation, and infrared spectroscopy. He holds a PhD from the University of Heidelberg, Germany, obtained in 2004, and an MSc from the University of Szeged, Hungary, earned in 2000. In addition to his role in optical sciences, he is also a Professor of Astronomy and Planetary Sciences. His work focuses on advancing observational technologies and methodologies to explore space and planetary environments, contributing to the development of innovative tools for astronomical research.

Research topics

• Physics
• Artificial Intelligence
• Astrophysics
• Computer Science
• Astrobiology
• Algorithm
• Geology
• Optics
• Astronomy

Selected publications

• The effect of stellar contamination on low-resolution transmission spectroscopy: needs identified by NASA’s Exoplanet Exploration Program Study Analysis Group 21

  RAS Techniques and Instruments · 2023 · 84 citations

  • Physics
  • Astronomy
  • Astrophysics

  Abstract Study Analysis Group 21 (SAG21) of NASA’s Exoplanet Exploration Program Analysis Group was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, plages, granules, and flares. This SAG brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current research needs that can be addressed in this context to make the most of transit studies from current NASA facilities like Hubble Space Telescope and JWST. The analysis produced 14 findings, which fall into three science themes encompassing (i) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities (‘The Sun as the Stellar Benchmark’), (ii) how stars other than the Sun extend our knowledge of heterogeneities (‘Surface Heterogeneities of Other Stars’), and (iii) how to incorporate information gathered for the Sun and other stars into transit studies (‘Mapping Stellar Knowledge to Transit Studies’). In this invited review, we largely reproduce the final report of SAG21 as a contribution to the peer-reviewed literature.

  DOI

• ACCESS and LRG-BEASTS: A Precise New Optical Transmission Spectrum of the Ultrahot Jupiter WASP-103b

  The Astronomical Journal · 2021 · 73 citations

  • Artificial Intelligence
  • Computer Science
  • Algorithm

  Abstract We present a new ground-based optical transmission spectrum of the ultrahot Jupiter WASP-103b ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>eq</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2484</mml:mn> </mml:math> K). Our transmission spectrum is the result of combining five new transits from the ACCESS survey and two new transits from the LRG-BEASTS survey with a reanalysis of three archival Gemini/GMOS transits and one VLT/FORS2 transit. Our combined 11-transit transmission spectrum covers a wavelength range of 3900–9450 Å with a median uncertainty in the transit depth of 148 parts per million, which is less than one atmospheric scale height of the planet. In our retrieval analysis of WASP-103b’s combined optical and infrared transmission spectrum, we find strong evidence for unocculted bright regions (4.3 σ ) and weak evidence for H 2 O ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.9</mml:mn> <mml:mi>σ</mml:mi> </mml:math> ), HCN ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.7</mml:mn> <mml:mi>σ</mml:mi> </mml:math> ), and TiO ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.1</mml:mn> <mml:mi>σ</mml:mi> </mml:math> ), which could be responsible for WASP-103b’s observed temperature inversion. Our optical transmission spectrum shows significant structure that is in excellent agreement with the extensively studied ultrahot Jupiter WASP-121b, for which the presence of VO has been inferred. For WASP-103b, we find that VO can only provide a reasonable fit to the data if its abundance is implausibly high and we do not account for stellar activity. Our results highlight the precision that can be achieved by ground-based observations and the impacts that stellar activity from F-type stars can have on the interpretation of exoplanet transmission spectra.

  DOI

• Helios-r2 : A New Bayesian, Open-source Retrieval Model for Brown Dwarfs and Exoplanet Atmospheres

  The Astrophysical Journal · 2020 · 69 citations

  • Physics
  • Astrophysics
  • Optics

  We present an improved, hybrid CPU-GPU atmospheric retrieval code, Helios-r2, which is applicable to medium-resolution emission spectra of brown dwarfs, in preparation for precision atmospheric spectroscopy in the era of the James Webb Space Telescope. The model is available as open-source code on the Exoclimes Simulation Platform. We subject Helios-r2 to a battery of tests of varying difficulty. The simplest test involves a mock retrieval on a forward model generated using the same radiative transfer technique, the same implementation of opacities, and the same chemistry model. The least trivial test involves a mock retrieval on synthetic spectra from the Sonora model grid, which uses a different radiative transfer technique, a different implementation of opacities, and a different chemistry model. A calibration factor, which is included to capture uncertainties in the brown dwarf radius, distance to the brown dwarf and flux calibration of the spectrum, may compensate, sometimes erroneously, for discrepancies in modeling choices and implementation. We analyze spectra of the benchmark brown dwarf GJ 570 D and the binary brown dwarf companions in the Epsilon Indi system. The retrieved surface gravities are consistent with previous studies and/or values inferred from dynamical masses (for Epsilon Indi Ba and Bb only). There remains no clear criterion on how to reject unphysical values of the retrieved brown dwarf radii. The inferred radii and corresponding masses should be taken with great caution. The retrieved carbon-to-oxygen ratios and metallicity depend on whether chemical equilibrium is assumed.

  DOI

Frequent coauthors

• Ilaria Pascucci

  University of Arizona

  158 shared

• Benjamin V. Rackham

  100 shared

• Mark S. Marley

  University of Arizona

  97 shared

• Ben W. P. Lew

  96 shared

• Adam J. Burgasser

  93 shared

• Andrés Jordán

  Millennium Institute of Astrophysics

  91 shared

• Alex Bixel

  83 shared

• L. R. Bedin

  80 shared

Education

• PhD

  University of Heidelberg

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